99W

99W

217

217

26 Sylvan Creek

River

Lake Oswego

Ash Creek

Summer Creek

Fa nno Creek

Tualatin

14206950

14206900

0 1 2 Kilometers

0 1 2 Miles

N

USGS streamflow gaging station

EXPLANATION

Watershed boundary

BEAVERTON

PORTLAND

TIGARD

METZGER

DURHAM

OREGON

Study Area

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2015–3003

January 2015

Organic Matters

Investigating the Sources, Transport, and Fate of 

Organic Matter in Fanno Creek, Oregon

Prepared in cooperation with Clean Water Services

The term organic matter refers to the remnants of all living 

material. This can include fallen leaves, yard waste, animal 

waste, downed timber, or the remains of any other plant 

and animal life. Organic matter is abundant both on land 

and in water. Investigating organic matter is necessary for 

understanding the fate and transport of carbon (a major 

constituent of organic matter).

Organic matter is necessary for maintaining a healthy 

ecosystem. It participates in a wide range of ecological 

functions, such as supplying food to the microbes that are part 

of the foundation of the food chain. Organic matter also plays 

a role in many other natural functions, including the binding 

and transport of some trace metals and controlling how light is 

absorbed in the water column. Organic matter in a stream can 

be found in many places, such as in the leaves that have fallen 

from a tree (termed “leaf litter”), in algae floating in the stream 

or attached to rocks, as part of the soil, or even suspended or 

dissolved in the water. 

The U.S. Geological Survey (USGS), in cooperation with Clean 

Water Services, recently completed an investigation into the 

sources, transport, and fate of organic matter in the Fanno 

Creek watershed. The information provided by this investigation 

will help resource managers to implement strategies aimed at 

decreasing the excess supply of organic matter that contributes 

to low dissolved-oxygen levels in Fanno Creek and downstream 

in the Tualatin River during summer. This fact sheet summarizes 

the findings of the investigation.

Organic Matter and the Environment

Organic matter plays a vital role in the environment, 

especially in the ecological health of a river system. It provides 

food for animals to eat. It supplies nutrients to the soil that 

plants need to grow. However, in some situations it can 

adversely affect the health of a river. When microbes break 

down organic matter in a stream, they use some of the oxygen 

that is dissolved in the water. This decreased dissolved oxygen 

in the water can stress fish and other aquatic animals that 

depend on oxygen to breathe. 

Some streams such as Fanno Creek in the Portland, Oregon,

metropolitan area (fig. 1) are listed as impaired waterways 

Figure 1. Location of Fanno Creek, Oregon.

4.0

6.0

8.0

10.0

Dissolved oxygen standard

12.0

A. 30-day mean dissolved oxygen, no credit for supersaturation

Dissolved oxygen 

concentration (mg/L)

May 16

June 08

July 01

July 24

Aug. 16

Sept. 08

Oct. 01

Oct. 24

Nov. 16

Dec. 09

Dec. 31

10–90 percentile

25–75 percentile

Median

EXPLANATION

Jan. 01

Jan. 17

Feb. 02

Feb. 18

Mar. 05

Mar. 21

Apr. 06

Apr. 22

May 08

6.0

8.0

10.0

12.0

B. 7-day mean dissolved oxygen, no credit for supersaturation

Dissolved oxygen standard

Figure 2. Dissolved oxygen (DO) concentration in Fanno Creek between 2003 and 2014. The DO standard line indicates two of the minimum 

thresholds to protect fish at different times in their life: (A) 6.5 mg/L—30-day average of daily mean DO (minimum criterion for rearing and 

migration period of May 16 to December 31) and (B) 11 mg/L—7-day average daily mean DO (minimum criterion for spawning period of 

January 1 to May 15).
Sediment erosion was observed using metal stakes hammered 

into the streambanks and measured after high flow events.

by the Oregon Department of Environmental Quality and 

require active management in order to improve dissolvedoxygen conditions and meet State standards. The standards 

vary seasonally and are based on whether fish are rearing or 

spawning (fig. 2). For Fanno Creek, the primary sources of 

organic matter are terrestrial (land based), such as leaf litter 

and soils, rather than aquatic, such as algae. So, how much 

terrestrial organic matter is available along the stream? To find 

the answer, USGS scientists began by measuring the amount of 

leaf litter, or foliage, available.

Organic matter in Fanno Creek comes mostly 

from terrestrial sources, such as leaf litter.

Leaf Litter (Foliage) Biomass

To estimate how much leafy material exists along 

Fanno Creek, USGS scientists used a combination of field 

measurements and remotely sensed data to calculate the foliage 

biomass, or the total dry weight of all leaf material (fig. 3). 

The annual leaf litter biomass was determined for two areas: 

(1) all foliage across the entire floodplain, and (2) only foliage 

overhanging the stream. Based on this analysis, an estimated 

990 metric tons (t) of organic matter is produced annually above 

the Fanno Creek floodplain, with about 140 t present directly 

over the channel. The amount of leaf material that falls along 

the floodplain varies depending on the size and age of the trees, 

and the density of the tree canopy. Because forest-dominant 

reaches along Fanno Creek contain more mature deciduous 

trees with broader tree canopies than either wetland or urbandominant reaches, they typically generate more leafy biomass in 

any given year.

Figure 3. Three-dimensional (3-D) view of tree canopy as 

represented by remotely sensed data (light detection and 

ranging [lidar]) near Durham City Park, Oregon. Lidar from 

Fusion software available at 

http://forsys.cfr.washington.edu/

fusion/fusionlatest.html.)

Deposition plates (30 x 30 centimeters) 

along the floodplain were routinely 

buried by sediment during high flow 

events.

USGS research explored organic matter 

“above, around, and in” Fanno Creek.

Organic Matter Transport with Fine Sediment

USGS scientists also assessed the organic matter (and 

carbon) contribution in the fine sediments along the floodplain 

and streambanks of Fanno Creek (fig. 4). Based on the carbon 

content of samples collected from the streambanks and 

measured rates of erosion and deposition, it was estimated that 

streambanks supplied between 50 and 120 t of organic matter 

annually during the study period. Generally, the organic matter 

stored in topsoil floodplain deposits was 2–5 times richer 

in carbon than that sampled directly from the subsoil of the 

streambanks. Comparatively, the streambanks supplied less 

organic matter to Fanno Creek than that measured from the 

foliage above.
AAXXXX_fig 01 Organic matter, in tonnes

-75

-50

-25

25

50

Downstream Upstream

0

Maximum estimate

Minimum estimate

EXPLANATION

Organic matter

Figure 4. Organic matter contribution from fine sediments along 

Fanno Creek. Positive values indicate overall deposition or storage; 

negative values indicate erosion or export. Net deposition is 

greatest along the middle sections of Fanno Creek where wetlands 

have been restored

Organic Matter as Instream Carbon

In addition to estimating organic matter contributions from 

above and along Fanno Creek, USGS scientists also assessed 

the instream movement and characteristics of organic matter. 

Using cutting-edge instream water-quality instruments (optical 

fluorescence sensors) and laboratory analyses (fluorescence 

spectroscopy and isotope chemistry), investigators estimated 

the amount and characteristics of dissolved, particulate, and 

total organic carbon in the stream. Each year, about 320 t of 

organic carbon floats or flows through Fanno Creek, with 

about 70 percent of organic carbon dissolved in the water. The 

concentration of the dissolved organic carbon varies throughout 

the year, but generally amounts to at least 3–4 milligrams per 

liter (mg/L) during most of the year. The instream and laboratory 

analyses confirm previous findings that the carbon in the stream 

predominantly originates from terrestrial organic matter, such as 

from leaf litter or other surface-soil runoff.

Water sample being collected by 

USGS personnel from Fanno Creek.

Mapping Erosion and Deposition

Based on geomorphic maps of the floodplain (fig. 5), USGS scientists have identified where sediment 

is being eroded and deposited along Fanno Creek. The maps are useful resources for those who plan and 

restore wetlands or riparian areas. Approximately 70 percent of the creek’s banks appeared highly erodible 

and resulted in 550–3,300 t of sediment being exported from Fanno Creek during the year-long study.

0 250 500 Feet

0 50 100 Meters

Feature

Bank

Bar

Culvert

Dam/Blockage

Floodplain

Low bench

Reinforced

Secondary water

Valley wall

Wetted channel

Erosion

Fluvial

Mass wasting

NA

None/Minimal

Other

Deposition

Fluvial

Mass wasting

NA

None/Minimal

Other

Base maps modified from Oregon Department of Geology and Mineral

Industries digital data, 1-meter resolution. UTM Zone 10, NAD83

A. Features B. Erosion C. Deposition

Channel blocked

by large wood

“Other” hillslope or 

storm drain erosion

Ponds on highelevation floodplain

retain organic 

material and runoff

Mass wasting 

at a cut bank

No erosion at

bank reinforced

by riprap

No deposition on

high-elevation

floodplain

Deposition on

low-elevation

floodplain

Mass wasting

deposit below

 a cut bank

Ponded water

on floodplain

Low-bench in 

active channel

Reinforced

banks

Channel blocked

by large wood

“Other” hillslope or 

storm drain erosion

Ponds on highelevation floodplain

retain organic 

material and runoff

Mass wasting 

at a cut bank

No erosion at

bank reinforced

by riprap

No deposition on

high-elevation

floodplain

Deposition on

low-elevation

floodplain

Mass wasting

deposit below

 a cut bank

Ponded water

on floodplain

Low-bench in 

active channel

Reinforced

banks

Figure 5. Examples of (A) channel features, (B) erosion areas, and (C) deposition areas mapped by USGS 

scientists near Fanno Creek Park in Tigard, Oregon.
Organic Matter Transport Following the First Storm of Autumn

In mid-October 2012, the first autumn storm swept across 

the Fanno Creek watershed. This first rain disturbed fine 

particulate materials that had been accumulating during spring 

and summer along the streambed and washed some materials 

into the stream from nearby storm systems and soils (fig. 6). 

Real-time water-quality monitoring equipment recorded the 

event (fig. 7). Due to the mobilization of this accumulated 

material, a large part of the annual supply of organic matter 

was transported during this storm, despite the fact that bigger 

storms occurred later in the year. Not only was the proportion 

of the organic matter greater during this first storm, but the 

transported material also was characteristically different than 

other storms. The organic matter showed more decomposition 

through microbial activity, reinforcing the conclusion that 

easy-to-mobilize sediment and decomposing organic matter 

had accumulated in the streambed during the dry summer 

months. Downstream in the Tualatin River, this storm 

plume containing decomposing organic matter resulted in 

a measurable and significant decrease in dissolved oxygen, 

a condition that tends to occur with the first autumn storms 

in most years and is a potential water-quality concern for 

resource managers.

Summer

Autumn flush

-2

-1

0

1

2

3

4

5

-5 -4 -3 -2 -1 0 1 2 3 4

Principal component 2

Principal component 1

Figure 6. Statistical analysis of water-quality samples collected 

from Fanno Creek. The two sample periods (summer low flow 

and the first autumn storm) were characteristically different from 

other samples collected.

0

5

10

15

20

25

30

Dissolved and total 

organic carbon

(mg/L) and discharge (m3/s)

4.0

5.0

6.0

7.0

8.0

Dissolved oxygen (mg/L)

4.5

5.5

6.5

7.5

0

50

100

150

200

250

Turbidity (FNU)

0

2

4

6

8

10

12

14

12 13 14 15 16 17 18

October 2012

A. First storm of autumn: Fanno Creek

B. Effects of first storm of autumn: Tualatin River 

Turbidity 

EXPLANATION

Total organic carbon Discharge 

Dissolved organic carbon Dissolved oxygen

Figure 7. First autumn storm readings for Fanno Creek and 

downstream in the Tualatin River. (A) Turbidity, total organic 

carbon, discharge, and dissolved organic carbon patterns in Fanno 

Creek in October 2012. Arrow indicates the beginning of the storm. 

(B) Dissolved oxygen and turbidity in the Tualatin River. The effects 

of the storm plume show up downstream about 1.5 days later.

Implications for Watershed Management

Fanno Creek flows through a heavily urbanized part of the Portland, Oregon area. As such, the watershed has many interested 

stakeholders, including municipalities, counties, utility providers, conservation groups, and private citizens. The health and safety 

of the creek has implications to more than just the aquatic life in the stream, but also to people who live and play along the creek. 

Therefore, it is useful to understand the critical water-quality issues in Fanno Creek and provide the maps, data, and insights necessary 

to assist resource managers to make decisions regarding management and restoration activities to maintain and enhance the 

stream environment.

Full Results of the Study

The results of the Fanno Creek study have been published in the 

Journal of Hydrology as a series of three articles. Figures presented 

in this fact sheet were modified from these articles.

Sobieszczyk, Steven, Keith, M.K., Rounds, S.A., and Goldman, J.H., 

2014, Investigating organic matter in Fanno Creek, Oregon, part 1 

of 3—Estimating annual foliar biomass for a deciduous dominant 

urban riparian corridor: Journal of Hydrology, v. 519D, p. 3001–

3009, http://dx.doi.org/10.1016/j.jhydrol.2014.06.054.

Keith, M.K., Sobieszczyk, Steven, Goldman, J.H., and Rounds, S.A., 

2014, Investigating organic matter in Fanno Creek, Oregon, part 

2 of 3—Sources, sinks, and transport of organic matter with fine 

sediment: Journal of Hydrology, v. 519D, p. 3010–3027, http://

dx.doi.org/10.1016/j.jhydrol.2014.07.027.

Goldman, J.H., Rounds, S.A., Keith, M.K., and Sobieszczyk, Steven, 

2014, Investigating organic matter in Fanno Creek, Oregon, part 3 

of 3—Identifying and quantifying sources of organic matter to an 

urban stream: Journal of Hydrology, v. 519D, p. 3028–3041, http://

dx.doi.org/10.1016/j.jhydrol.2014.07.033.

Publishing support provided by the 

U.S. Geological Survey, Tacoma Publishing Service Center

Authors: Steven Sobieszczyk, Mackenzie K. Keith, Jami H. Goldman, and

Stewart A. Rounds

For information contact:

Director, Oregon Water Science Center

U.S. Geological Survey

2130 SW 5th Ave., Portland, OR 97201

http://or.water.usgs.gov/

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20153003